1. Field of the Invention
This invention concerns the analysis of samples of neat cement and blends containing a cement component. In particular, the analysis provides a method with which to quantify the various mineral phases in such samples, and is applicable to dry powdered samples of cements such as might be used in the construction industry or oil well operations.
2. Description of the Related Art
Oil-well cementing operations are either "primary", done in the course of drilling a well, or "secondary" or "remedial", intended to remedy deficiencies in primary cementing or alter the well completion for production. During a "primary" cementing process, a cement slurry is pumped down the steel casing and up the annulus between the casing and the surrounding rock formation. The cement slurry must remain sufficiently mobile for the pumping operation to be completed. After it has been placed in the annulus, the cement hardens to form a hydraulic seal in the wellbore, preventing the migration of formation fluids in the annulus. The set cement sheath provides support to the casing string and protects the casing against corrosion by formation fluids. Examples of "secondary" or "remedial" cementing processes are "squeeze" cementing, during which a cement slurry is forced through holes or splits in the casing into voids or a porous rock formation and "plug" cementing, during which a relatively small volume of cement slurry is placed in the wellbore in order to prevent the loss of drilling fluid during the drilling phase or to seal off a depleted zone during the production phase.
The basic ingredient in current cement compositions is portland cement. The raw ingredients of portland cement are lime (CaO), silica (SiO.sub.2), alumina (Al.sub.2 O.sub.3) and iron oxide (Fe.sub.2 O.sub.3). Lime is obtained from calcareous rock deposits and industrial alkali waste products. Silica, alumina and iron oxide are derived from argillaceous materials such as clays, shales and marls and from artificial sources such as blast furnace slag or fly-ash waste from coal-fired power stations. The diversity of raw materials used to manufacture cements contributes to the wide range of composition of the final products. A pulverised blend of the raw materials is fed into a rotating kiln where temperatures as high as 1500.degree. C. produce a molten mixture; subsequent cooling induces a complex series of reactions to produce the four principal mineral phases: "Alite" which is a tricalcium silicate, Ca.sub.3 SiO.sub.5 (commonly abbreviated to C.sub.3 S, which devotes three moles of CAO and one mole of SiO.sub.2); "Belite" which is a dicalcium silicate, Ca.sub.2 SiO.sub.4 (C.sub.2 S); "Aluminate" which is tricalcium aluminate, Ca.sub. 3 Al.sub.2 O.sub.6 (C.sub.3 A); and "Ferrite" which is tetracalcium aluminoferrite, Ca.sub.4 Al.sub.2 Fe.sub.2 O.sub.10 (C.sub.4 AF). These four principal mineral phases leave the kiln as a "clinker" which is subsequently ground with gypsum CaSO.sub.4.2H.sub.2 O to produce the finished portland cement product.
The finished portland cement product may contain a variety of sulphate minerals which are produced during the manufacturing process. The added gypsum may dehydrate to bassanite and/or anhydrite during grinding, and it may also react with alkali sulphate to produce syngenite. The concentration and form of the minor sulphate and hydroxide/carbonate components of a cement may have a considerable effect on the slurry performance. For example, the presence of syngenite in oil well cement may cause premature or "false" setting of the slurry.
Specifications for oil-well cements have been established by the American Petroleum Institute (API). There are currently eight classes of API Portland cement, designated Class A through to H which are classified according to the depths to which they are placed, and the temperatures and pressures to which they am exposed. The main chemical criterion for classifying Portland cements is the relative distribution of the four main clinker phases, known as the "potential phase composition". The most widely accepted method of expressing the relative amounts of the principal clinker phases relies upon a series of calculations based on an oxide analysis of the cement sample. This method, based upon various phase equilibria relationships between the cement components, was first introduced in 1929 by R. H. Bogue in his publication, "Calculation of the compounds in portland cement". For each Class of API Portland cement, concentration limits for the Bogue phases, C.sub.3 S, C.sub.2 S C.sub.3 A and C.sub.4 AF are specified; in addition, limits on the amounts of the alkalis, free CaO, MgO, SO.sub.3, insoluble residue and loss on ignition are specified. Other physical parameters which appear in the API specification include the fineness of the cement powder, and the performance of the cement slurry and set cement according to standard tests. The performance tests include measurements of thickening time, compressive strength, expansion and free water.
Whilst the Bogue method of expressing the principal clinker phases in a cement sample has remained the industry standard for many years, it has certain limitations which have lead to the proposal of a "modified Bogue" procedure which may be used to calculate a more realistic "potential phase composition" from a full oxide analysis of a clinker or cement sample.
Infrared spectroscopy has been previously applied to the analysis of cement powders and blends. The use of infrared spectroscopy as a tool for ascertaining the forms of calcium sulphate present (gypsum, hemihydrate, soluble anhydrite, insoluble anhydrite) and whether any hydration (prehydration coupled with carbonation) has occurred or not has been proposed, see II Cemento,1,35-46 (1987). No attempt has been made to deconvolve the infrared spectra with a view to obtaining quantitative analyses of either the minor sulphate and carbonate phases or the principal clinker phases.
Paper WHC-SA-0493-FP prepared for the US Department of Energy, Assistant Secretary for Defense Programs, July 1989, Rebagy, TV and Dodd, DA describes a method to determine the components of cement blends. The proposed method involves the collection of diffuse reflectance FTIR spectra for cement blend samples; the concentration of the components in the blends are determined by using a sequential spectral subtraction program using the spectra of the pure components. Quantitative data pertaining to the analysis of cement, blast furnace slag and fly ash in a 3-component blend and to the analysis of cement, fly ash, attapulgite clay and indian red pottery clay in a 4-component blend are given. The method treats the cement component of the blends as a single component and does not propose a quantification of the individual major and/or minor phases of the cement component of the blend.